Improvements relating to reproduction arrangements for stereophonic sound systems



Sept. 25, 1962 w. s. PERcIvAL 3,055,930

IMPROVEMENTS RELATING TO REPRODUCTION ARRANGEMENTS FOR STEREOPHONICSOUND SYSTEMS Filed Oct. 9, 1959 1 AMPLIFYING AND VOLTAGE- TO-CURRENTglgfggggLATlNG AMPLIFIER AUDIO AMPLIFIER HALL AMPLIFIERS (PHASEINVERTING) CRYSTAL FIG 1 RELAY AMPLIFIER (PHASE INVERTING) IMPROVEMENTSRELATING T REPRODUCTHGN AGEMENTS FOR STEREUPHGNIC GUND SYSTEMS WilliamSpencer Percival, West Ealing, London, "England, assignor to Electric 81Musical Industries Limited, Hayes, England, a company of Great BritainFiled Oct. 9, 1959, Ser. No. 845,469 Claims priority, application GreatBritain Get. 11, 1955 3 Claims. (Cl. 179-1) This invention relates toreproduction arrangements for stereophonic sound systems.

One defect of stereophonic reproduction in which audio signals arereproduced by two spaced loudspeakers is that as the listener movesalong the line parallel to the line joining the loudspeakers, theapparent position of the source of sound tends also to move. The resultis that satisfactory stereophonic reproduction is available only forlisteners on or near the centre line between the loudspeakers.

The object of the present invention is to provide an improvedstereophonic sound reproducer which enables stereophonic reproduction tobe obtained such that the apparent direction of reproduced sound is lessdependent on the position of the listener than in arrangements such asindicated in the preceding paragraphs.

According to the present invention there is provided a stereophonicsound reproducer comprising means for providing an audio signalrepresenting the waveshape of sound, means for providing a controlsignal which represents the direction or position of the source orsources of said sound, and is substantially independent of the loud nessof the sound, at least three loudspeaker units, and means for applyingthe audio signals to said loudspeaker units in relative proportionsdependent upon the instantaneous value of said control signal, in such away that the audio signal is applied to a single and different one ofsaid loudspeaker units for different discrete values of said controlsignal and is proportioned between the respective two loudspeaker unitsfor values of the control signal intermediate said discrete values.

The expression loudspeaker unit is used to denote either a singleloudspeaker, or a group of loudspeakers, having for example differentfrequency responses, mounted in such relative positions that soundemanating from them would appear to the listener to come substantiallyfrom the same direction or position.

The invention may be applied to reproducers adapted to reproduce soundsfrom a radio broadcast comprising the audio signal and the controlsignal, in which case the means for providing the audio signal and thecontrol signal may comprise amplifying and detecting circuits such asare conventional in broadcast radio receivers. The invention maynevertheless be applied to reproducers adapted to reproduce signalsderived from twin track record tape or complex cut record discs. In sucha case the input signal to the reproducer may comprise two audiosignals; for example the sum and difference of two directional signalsand the means for providing the control signal at the reproducer wouldneed to be such as to derive the latter signals from the two audiosignals.

In order that the present invention may be clearly understood andreadily carried into elfect the invention will now be described withreference to the accompanying drawings, in which:

FIGURE 1 illustrates diagrammatically, and mainly in block form, oneexample of a stereophonic sound reproducer according to the presentinvention, and

FIGURE 2 illustrates a modification of FIGURE 1.

It will be assumed that the sound reproducer which is illustrated inFIGURE 1 is adapted to receive a broadcast carrier wave which ismodulated in any suitable manner 3,055,980 Patented Sept. 25, 1962 ireThe audio signal is denoted by the symbol V whilst the control signal isdenoted by the symbol VI!L+VIIR The control signal, which for brevitymay be denoted as V is substantially independent of the loudness of thesound, it has the value of +1 when it represents sound coming from theextreme left of the stereophonic base, it has the value 1 when itrepresents sound coming from the extreme right of the stereophonic base,and it has value zero when it represents sound coming from the centre ofthe stereophonic base. The control signal and the audio signal may forexample be obtained at the transmitter by means of apparatus such asdescribed in United States patent application Number 742,175, filed byW. S. Percival on June 16, 1958. This application describes thederivation of a control signal having the form and then changed to theother form, if necessary, at the receiver.

The receiver shown in FIGURE 1 comprises an antenna 1 which feeds thereceived carrier wave to conventional I represent the extreme rightdirection.

T right of centre yield zero output voltage.

amplifying and demodulating circuits 2 which are arranged to separatethe audio signal from the control signal V The signals V and V asderived from the cir cuits Z are voltage signals and the polarity of theaudio signal is such that it can be represented as V The audio signal -Vis applied to one end of a resistor 3 which will be referred tosubsequently and it is also ap plied to an amplifier 4- which isarranged to derive a current i proportional to the voltage V but is ofpositive polarity. The relationship between a voltage signal V anddirection as represented by the diagram 5 which shows that V has amaximum positive value L to represent the extreme left direction, haszero value to represent the central direction, and has a maximumnegative value R to The signal V is applied to a bridge network of fourdiodes 6, 7, 8 and 9, polarised as shown so that at the junction of thediodes 6 and '7 signals representing left of centre yield a positivevoltage whereas central signals and those representing On the otherhand, at the junction of the diodes 8 and 9, signals representing rightof centre give a negative output voltage whereas central signals andsignals representing left of centre give zero output voltage. Therelationships between direction and voltage of the two output signalsderived from the diode bridge circuit are represented by the diagrams 1tand 11. The voltage derived from the junction of 6 and 7 is applied to anon-phase inverting amplifier 12 and that derived from the junction ofthe diodes 8 and 9 is applied to a' phase inverting amplifier 13 and thetwo signals are then applied to the field windings 14 and 15 of two Hallamplitude circuits, comprising Hall crystals represented by therectangles 16 and 17. It will be understood that the field windings 14and 15 are arranged to set up magnetic fields normal to the planes ofthe respective Hall crystals. The current i from the aforesaid amplifier4 is fed through the'Hall crystals from one edge to another so that thecurrent flows in a direction perpendicular to the respective magneticfields. As a result, voltages are induced between the remaining edges ofthe Hall crystals, proportional to control current i and the respectivemagnetic fields. As shown the voltage induced across the Hall crystal 16is applied to one end of the resistor 18 and through an audio amplifier19 to a loudspeaker 20 which is placed at the left hand side of theauditorium. Similarly the voltage induced across the Hall crystal 17 isapplied to one end of the resistor 21 and through an audio amplifier 22to a loudspeaker 23 which is placed at the right hand side of theauditorium. The other ends of the resistors 18 and 21, of the aforesaidresistor 3 are joined together, and any voltage appearing at thisjunction is applied via audio amplifier 25 to a loudspeaker 26 which isplaced in the centre of the auditorium. The resistors 3, 18 and 21 haveequal resistances and the constant of proportionality between thevoltage V applied to the resistor 3 and the current i is such that V isnumerically equal to the maximum voltages obtained from the outputs ofthe Hall crystals 16 and 17. The voltage V is however of oppositepolarity to the outputs of the Hall crystals. The amplifier 25 isrequired to be phase inverting relative to the amplifiers 19 and 22, andto have three times as much gain, in order to compensate for attenuationof the signal V at the junction of 3, 18 and 21.

The result of the arrangement shown is that the output of the Hallcrystal 16 is a voltage proportional to both the audio signal V and thecontrol voltage obtained from the amplifier 12, and conforming to thedirectional relationship 10. The output of the Hall crystal 17 is avoltage proportional to both the audio signal V and the control voltageobtained from the amplifier 13, and conforming to the inverteddirectional relationship 11. The voltage output from the Hall crystal 16will equal to the voltage V when the direction is fully left, and inthis condition because the resistors 3, 18 and 21 have equalresistances, and because the output impedance of a Hall crystal can beassumed to be small, no voltage will appear at the input to theamplifier 25 or at the input to the amplifier 22. Therefore sound isreproduced only by the loudspeaker 20. Similarly when the direction isfully right, sound is reproduced only by the loudspeaker 23. For acentral direction, the output voltage of both Hall crystals is zero andsound is reproduced only from the central loudspeaker 26. For directionsbetween left and centre, sound is reproduced simultaneously from theloudspeakers 20 and 26, the audio signal being in effect applied tothese two loudspeakers in different ratios dependent upon theinstantaneous value of the control signal. For a given sound of constantvalue moving from left to centre the output of the two speakers 20 and26 is also of constant volume. No sound is reproduced from the right endloudspeaker 23. Conversely for directions between centre and right,sound is reproduced from the loudspeakers 23 and 26 in different ratiosdependent upon the values of the control signal, no sound beingreproduced by the left hand loudspeaker 20.

In the modification of the invention which is illustrated in FIGURE 2,one of the Hall crystal amplitude control circuits and one of the audioamplifiers before the loudspeakers are dispensed with. The controlsignal V A is applied directly to the amplifiers 12 and 13, theamplifier 13 being as in FIGURE 1 a phase inverting amplifier. Theoutputs of the amplifiers 12 and 13 are then applied to diodes 30 and 31which have a common load resistor 32, so that the voltage across theresistor 32 corresponds to the more positive of the voltages applied tothe diodes 30 and 31. The voltage across the resistor 32 thereforerepresents the modulus of the voltage V the relationship between voltageand direction being as indicated by the diagram 33. This voltage isapplied to the field winding 34 of the single Hall crystal amplitudecontrol circuit the crystal of which is denoted by the reference 35. Thecurrent L, is fed to this crystal in the manner described with referenceto FIGURE 1 and the resultant voltage is applied to one end of theresistor 36 which corresponds either to the resistance 18 or 21 ofFIGURE 1. The other end of this resistor is joined as shown to theresistor 3. The output voltage from the Hall crystal 35 is also appliedto an audio amplifier 37. The output of the amplifier 37 is applied to aswitch 38 which though represented as a mechanical switch may in fact bean electronic switch. The switch is controlled by a polarized relay 39in such a way that the output of the amplifier 37 can be applied eitherto the left hand loudspeaker 20 or to the right hand loudspeaker 23. Thevoltage at the junction of the resistors 3 and 36 is applied via thephase inverting amplifier 25 to the central loudspeaker 26. The gain ofthe amplifier 25 is adjusted relative to that of the amplifier 37 tosuit the attenuation of the network 3, 36.

The relay 39 is polarized in such a way that whenever the controlvoltage V has a positive value, the output of the amplifier 37 isapplied to the left hand loudspeaker 20 and whenever the control voltageV has a negative value, the output of the amplifier 37 is applied to theright hand loudspeaker 23. The arrangement takes advantage of the factthat at any instant one or other of the loudspeakers 20 and 23 shouldreproduce no sound, and the operation of the relay 39 ensures that theswitching over from one loudspeaker to the other occurs when the controlvoltage corresponds to a centre direction, in which circumstance neitherof the loudspeakers 20 and 23 should reproduce sound. Distortion of thesound output due to switching is therefore avoided. In other respectsthe operation of FIGURE 2 is similar to that described with reference toFIGURE 1.

In both FIGURE 1 and FIGURE 2, the proportioning of the audio signalamong the loudspeakers is performed in effect by controlling theamplitude of the audio signal V in response to the modulus of thecontrol signal V to produce an amplitude controlled signal, and byderiving a signal representing the difference of the uncontrolled audiosignal V and the amplitude controlled signal, the differencing beingeffected in the network 3, 18 and 21 of FIGURE 1 and the network 3 and36 of FIGURE 2. The difference signal is applied to the centralloudspeaker and the amplitude controlled signal applied either to theloudspeaker 20 or the loudspeaker 23 depending on whether the controlsignal represents a direction or position to one side or other of thecentral direction or position. In the arrangement shown in FIGURE 1 twoamplitude controlling circuits are used to form the amplitude controlledsignal, the amplitude control circuit including the crystal 16 forcontrolling the audio signal with the control signal when the controlsignal has a positive value, and the amplitude control circuit includingthe crystal 17 for controlling the audio signal with the negative of thecontrol signal when the control signal has a negative value.

The control signal V requires to have only a limited band width relativeto the audio signal V although the control signal may extend in to theaudio frequency band. The band width of the control signal may be forexample from 0 to c./s. The amplifiers 12 and 13 therefore require onlya limited pass band, corresponding to that of the control signal. Theloudspeakers 20, 23 and 26 need not be single loudspeakers, but maycomprise groups of loudspeakers positioned in such a way as to formsingle loudspeaker units. Furthermore although the arrangementsdescribed ensure that the sum of the signals applied to the threeloudspeakers is always linearly proportional to the audio signal it maynot always be necessary to arrange that this is so, as the acoustics, ofa particular room may be such that better results are obtained with someother relationship. The amplitude controlled signals obtained from theamplitude control circuits may not be linearly related to the controlsignal, in order to obtain optimal conditions as judged by listeningtests. The reproducer may include means for varying the relationshipbetween the control signal V and direction, to provide for spreadadjustments which can be controlled by the listener.

'In a further modification, it is arranged that the very low audiofrequencies are diverted entirely to the amplifier 25 and the centralloudspeaker 26. In this case only the loudspeaker 26 need have a fullbass response. This modification takes advantage of the fact that stereoeffects for very low frequencies are not so important as for higherfrequencies and allows economy to be effected at the cost of only asmall loss of stereo effects since good bass reproduction involves moreexpensive amplifiers and speakers.

In the arrangement shown in FIGURE 2, the switch 33 may be placed beforethe amplifier 37, and in this case a separate audio amplifier must beintroduced for the speaker 23. In this modification the switch issituated at a point in which the signal level is relatively low and thisfacilititates the employment of an electronic switch. If an electronicswitch is used, that is one employing thermionic or transistor valves,the impedances of the valves when conducting are of low value so thatthe power consumption is considerably less than that required for anadditional amplifier.

The invention can also be applied to reproducers in which more thanthree loudspeaker units are provided, spaced one from another along thestereophonic base. In such an application of the invention, an expedientsimilar to that illustrated in FIGURE 2 is preferably adopted in orderto reduce the number of audio amplifiers which are required. The switchequivalent to 38 is required to operate in such a way that the two audioamplifiers are always connected to the two speakers which are requiredto operate at a given instant. The switching is arranged to occur atinstants when all the sound output is produced by one speaker and issuch as to switch the amplifier not in service at that instant from thespeaker on one side of the inoperative speaker to that on the otherside.

What I claim is:

1. A stereophonic sound reproducer comprising means for providing anaudio signal representing the waveshape of sound, means for providing acontrol signal which represents the direction or position of the sourceor sources of said sound, at least left hand, central and right handloudspeakers and means responsive to the instantaneous value of saidcontrol signal comprising means for controlling the amplitude of saidaudio signal in response to the modulus of said control signal toproduce an amplitude controlled signal, means for deriving a signalrepresenting the difference between said audio signal and said amplitudecontrolled signal, means for applying said difference signal to saidcentral loudspeaker, and means for applying said amplitude controlledsignal to said left hand or said right hand loudspeaker, depending onWhether the control signal represents a direction or position to theleft or right of a central direction or position.

2. A sound reproducer according to claim 1 comprising a first amplitudecontrol circuit for producing said amplitude controlled signal when thecontrol signal represents any direction or position to the left of saidcenter direction or position, and a second amplitude control circuit forproducing said amplitude controlled signal when the control signalrepresents any direction or position to the right of said centraldirection or position, said left hand loudspeaker being connected toreceive the amplitude controlled signal from said first amplitudecontrol circuit, and said right hand loudspeaker being connected toreceive the amplitude controlled signal from said second amplitudecontrol circuit.

3. A sound reproducer according to claim 1 comprising an amplitudecontrol circuit common to both said left hand and right handloudspeakers for producing said amplitude controlled signal, and aswitch for applying the amplitude controlled signal from said amplitudecontrol circuit either to the left hand or right hand loudspeakerdepending on whether said control sign-a1 represents a direction orposition to the left or the right of said central direction or position.

De Boer Apr. 10, 1946 Volkma-n June 14, 1960

